Structure of Triacylglycerols

Stereospecific analysis of milk fat fractions containing triacylglycerols of different molecular weight have shown that, for fatty acids of chain length C4 to Ci6, the general pattern of fatty acid distribution in normal milk fat is similar to the pattern of distribution in the triacylglycerol fractions of different 

As noted earlier, milk fat contains a very complex mixture of triacylglycerols. This complexity has made the identification and characterization of individual triacylglycerols extremely difficult. Moreover, the fact that no two batches of milk fat have exactly the same composition adds to the difficulty. As a result, the majority of the earlier studies were aimed at elucidating the general types of triacylglycerols present rather than obtaining quantitive data about individual triacylglycerols. 

The saturated and monoene triacylglycerol classes were dominant and each comprised about 35 to 40% of the total milk fat, while the approximate proportions of the high-, medium- and low-molecular weight fractions were 40, 20 and 40%, respectively. 

A number of recent investigations have shown that mass spectrometry (MS) is a rapid and effective method for the identification of triacylglycerol species of milk fat that are compositionally different (Myher et al., 1988, 1993 Laakso and Kallio, 1993 Spanos et al., 1995 Laakso and Manninen, 1997 Mottram and Evershed, 2001 Kalo et al., 2004). In fact, a range of mass spectral techniques, such as electron ionization, fast atom bombardment, chemical ionization, atmospheric pressure chemical ionization and electrospray MS, have been used to study triacylglycerols. The later three are soft ionizing techniques, which retain substantial amounts of the molecular ion, rather than fragmenting the molecule into a number of parts. These methods have allowed the determination of 

Mottram and Evershed (2001) undertook a similar study in which they fractionated milk fat by two different methods, silica TLC and gel 

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Structure of triacylglycerols (TAG) The three fatty acids esterified] to a glycerol molecule are usually not of the same type. The fefy I acid on carbon 1 is typically saturated, that on carbon 2 is typi-j cally unsaturated, and that on carbon 3 can be either. Recall thal the presence of the unsaturated fatty acid(s) decrease(s) thl I melting temperature of the lipid. An example of a TAG molecule H shown in Figure 16.12. 

Figure 4. Typical variations in the chain-length structures of triacylglycerol crystals. An arrow means a leaflet.

L.G.J. (1993) The crystal structure of triacylglycerol lipase from Pseudomonas glumae reveals a partially redundant catalytic aspartate. FEBS Lett. 331, 123-128... 

Noble, M. E. M., A. Cleasby, L. N. Johnson, M. R. Egmond, and L. G. J. Frenken. 1993. The Crystal Structure of Triacylglycerol Lipase from Pseudomonas Glumae Reveals a Partially Redundant Catalytic Aspartate. FEBS Letters 331 (1-2) 123-128. 

Ozerinina, O.V., Berezhnaya, GA, Yeliseev, I.P., and Vereshchagin, AG. Composition and structure of triacylglycerols from sea buckthorn fruit mesocarp, Prikl. Biokiiim. Mikrobiol. 24 (1988), 422-429. 

Batrakov, S.G. and Tolkachev, O.N. (1997) The structures of triacylglycerols from sclerotia of the rye ergot Claviceps purpurea (FRIES) TUL. Chem. Phys. Lipids, 86,1-12. 

Fig. 1. Structure of triacylglycerols (I), i.e. the typical reserve lipids of oil seeds, and of ionic and non-ionic polar lipids (II), such as glycerophospholipids and glycerogalactolipids, which function as membrane lipids. Wax esters (III) are only formed in jojoba (Simmondsia chinensis) seeds as energy reserves. (Rj, R2, R3 various acyl moieties R4 alkoxy moiety X various polar head groups of ionic and non-ionic membrane lipids)...

Figure 4.10 The structure of triacylglycerol and types offatty acids.

Figure 26.2 shows the structures of two typical triacylglycerols, 2-oleyl-l,3-distearylglycerol (Figure 26.2a) and tristearin (Figure 26.2b). Both occur naturally—in cocoa butter, for example. All three acyl groups in tristearin are stearyl (octadecanoyl) groups. In 2-oleyl-l,3-distearylglycerol, two of the acyl groups are stearyl, but the one...

Draw the structures of (a) all the possible triacylglycerols that can be formed from glycerol with stearic and arachidonic acid, and (b) all the phosphatidylserine isomers that can be formed from palmitic and linolenic acids. 

HDL and VLDL are assembled primarily in the endoplasmic reticulum of the liver (with smaller amounts produced in the intestine), whereas chylomicrons form in the intestine. LDL is not synthesized directly, but is made from VLDL. LDL appears to be the major circulatory complex for cholesterol and cholesterol esters. The primary task of chylomicrons is to transport triacylglycerols. Despite all this, it is extremely important to note that each of these lipoprotein classes contains some of each type of lipid. The relative amounts of HDL and LDL are important in the disposition of cholesterol in the body and in the development of arterial plaques (Figure 25.36). The structures of the various... 

Figure 2S-1. Generalized structure of a plasma lipoprotein. The similarities with the structure of the plasma membrane are to be noted. Small amounts of cholesteryl ester and triacylglycerol are to be found in the surface layer and a little free cholesterol in the core.

Using PTLC six major fractions of lipids (phospholipids, free sterols, free fatty acids, triacylglycerols, methyl esters, and sterol esters) were separated from the skin lipids of chicken to smdy the penetration responses of Schistosoma cercaria and Austrobilharzia variglandis [79a]. To determine the structure of nontoxic lipids in lipopolysaccharides of Salmonella typhimurium, monophosphoryl lipids were separated from these lipids using PTLC. The separated fractions were used in FAB-MS to determine [3-hydroxymyristic acid, lauric acid, and 3-hydroxymyristic acids [79b]. 

Certain classes of lipids are susceptible to degradation under specific conditions. For example, all ester-linked fatty acids in triacylglycerols, phospholipids, and sterol esters are released by mild acid or alkaline treatment, and somewhat harsher hydrolysis conditions release amide-bound fatty acids from sphingolipids. Enzymes that specifically hydrolyze certain lipids are also useful in the determination of lipid structure. Phospholipases A, C, and D (Fig. 10-15) each split particular bonds in phospholipids and yield products with characteristic solubilities and chromatographic behaviors. Phospholipase C, for example, releases a water-soluble phosphoryl alcohol (such as phosphocholine from phosphatidylcholine) and a chloroform-soluble diacylglycerol, each of which can be characterized separately to determine the structure of the intact phospholipid. The combination of specific hydrolysis with characterization of the products by thin-layer, gas-liquid, or high-performance liquid chromatography often allows determination of a lipid structure. 

Figure 21-5 A more complete outline of the biosynthesis of triacylglycerols, glycolipids, and phospholipids including characteristic eukaryotic pathways. Green lines indicate pathways utilized by both bacteria and eukaryotes. Structures of some of the compounds are shown in Fig. 21-4. The gray arrows show the formation of phosphatidylserine by exchange with ethano-lamine (Eq. 21-10).

Reversed-phase HPLC has been used to analyze the oxidation products of triacylglycerols in edible oils. The detection is often based on monitoring the conjugated dienes with an ultraviolet detector (234-235 nm). However, the UV detector provides no information about oxidation products without a conjugated diene structure, e.g., products of oleic acid. Information about these compounds is important when oils with a high oleic acid content are studied. The most common universal detector types—refractive index and flame ionization detectors—are not sensitive enough to detect small amounts of oxidation products. 

The structures of common lipids, (a) The structures of saturated and unsaturated fatty acids, represented here by stearic acid and oleic acid, (b) Three fatty acids covalently linked to glycerol by ester bonds form a triacylglycerol. (c) The general structure for a phospholipid consists of two fatty acids esterified to glycerol, which is linked through phosphate to a polar head group. The polar head group may be any one of several different compounds—for example, choline, serine, or ethanolamine. 

The structures of the various lipoproteins appear to be similar (figs. 20.11 and 20.12). Each of the lipoprotein classes contains a neutral lipid core composed of triacylglycerol and/or cholesteryl ester. Around this core is a coat of protein, phospholipid, and cholesterol, with the polar portions oriented toward the surface of the lipoprotein and the hydro-phobic parts associated with the neutral lipid core. The hydrophilic surface interacts with water in plasma, promoting the solubility of the lipoprotein. 

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